Reliability and speed of response are among the most important characteristics of scanning valves used in multi-point gas pressure measuring systems for aerodynamic tests. The apparatus used in the tests must meet contradictory requirements of increasing the number of pressure sensing points and reducing the size of the scanning valve arranged inside the wind-tunnel models.
Known in the art are scanning valves (see Scanivalve Inc. USA, General Catalog, Oct. 30, 1972) comprising a stator and a rotor pressed thereagainst with its end face. The stator has an outlet port in the center and inlet ports in the periphery. The inlet ports alternately communicate with the outlet passage made in the rotor, as the latter rotates relative to the stator. The outlet port communicates with a pressure transducer. The rotor is driven by a step drive.
In operation, gas pressure applied to the rotor urges it away from the stator, which action may unseal the space formed by the ports of the stator and the passage of the rotor.
To balance this force, a counterpressure is generated in the scanning valve inner space of a value approximately equal to the average value of gas pressure being measured and controlled at a given moment. The counterpressure is generated by means of a special pressure source which complicates the scanning valve construction and lowers its reliability.
The above scanning valves are made with 24 and 48 inlet ports. Further increase in the number of ports involves considerable technological difficulties and leads to a significant increase in the weight and size of the commutator. If a greater number of points must be pressure-sensed, the use is made of sets of several computer-controlled commutators.
The stepped rotation of the rotor is a source of increased dynamic loads on the scanning valve components, which limits the speed of response of the scanning valve.
Known in the art is a scanning valve (see USSR Inventor's Certificate No. 290188, Int. Cl. 2 G01 L 19/00) having a hollow cylindrical stator provided with a helical groove on the inner surface.
The helical groove has inlet ports through which pressure being sensed is applied to the interrogating electromagnetic valve installed on the axle of the rotor moving along the helical groove and connected to a pressure transducer by means of a coil pipe. The rotor is intermittently shifted axially along the stator by means of an electromagnetic drive having a ratchet-and-pawl gear and a differential reduction gear.
During commutation of one of the inlet ports the electromagnetic valve is pressed against the inner surface of the stator overcoming the force developed by gas pressure in this inlet port. Friction forces arising at the stator-to-rotor interface are such as to prevent sliding of the electromagnetic valve over the stator surface. For commutation of the next inlet port, the electromagnetic valve is switched off, moved away from the stator surface, turned by an electromagnetic drive and again pressed against the stator surface. Forces from the electromagnetic valve are taken up by the rotor axle.
As the gas pressure increases, in accordance with the above procedure, the force developed by the electromagnetic valve must be increased which results in an increased mass of the electromagnetic valve and inertia of the rotor.
The speed of response of the above scanning valve is relatively small because of the intermittent operation of the drive electromagnet and electromagnetic valves and due to considerable masses of intermittently moving assemblies.
An intricate design of the rotor causes an increase in the stator size, and consequently the whole scanning valve is bulky, which impedes its use in aerodynamic tests, particularly in model wind tunnel tests of aircraft.
Intermittent operation of the scanning valve assemblies reduces the reliability and service life of the scanning valve as a whole.
In the above scanning valve, the number of the inlet ports being sensed is determined by counting electric pulses fed to the electromagnetic valve. For this reason, in case of an eventual failure of the valve to operate the reliability of identification of the number of the scanning valve inlet ports being sensed and of the value of the measured gas pressure materially decreases.